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Atomic Mass Because the mass of a single atoms is so small, for convenience, chemists use the unit called atomic mass unit (amu) also known as dalton (Da). One amu is defined as 1/12 the mass of an atom of 12C and is equal to 1.66054 x 10 -24 g. Mass of one 12C atom = 12.000 amu Because electrons mass is negligible, the protons and neutrons each have a mass of ~1 amu An element s atomic mass (listed in Periodic Table) are weighted averages for the naturally occurring mixtures of different isotopes of that element Atomic mass of an element = (mass of each isotope x the fractional abundance of the isotope) Carbon has two natural isotopes, C-12 and C-13. At. mass of C = (fraction of 12C )(mass of 12C) + (fraction of 13C)(mass of 13C) Given that: mass of 12C = 12.000 amu, mass of 13C = 13.0034, and abundance of 12C = 0.9889, abundance of 13C = 0.0111 Atomic mass of C= (0.9889 x 12.000 amu) + (0.0111 x 13.0034) = 12.011 The element C in the Periodic Table is shown as: Problem ?: Copper has two isotopes: copper-63 (69.17%; isotope mass 62.94) and copper- 65 (30.83%; isotope mass 64.94 amu). Calculate the atomic mass of copper. Ans: 63.55 amu 1 Compounds Chemical Reactions- atoms from two or more different elements combine, creating new materials called compounds. Reactants (starting substances) Products (new compounds) O 2 + 2 H2 pure elements 2 H2 O water molecule H2O means two H atoms attached to one O atom Molecular Compounds 2 Molecular Compounds Covalent Bonding (in Molecules): The most common type of chemical bond is formed when two atoms share some of their electrons Naming Molecules Molecules are made up of two or more non-metals ( C, N, O, H, S, P, B, halogens, Se, Si) Naming Binary Molecular Compounds: 1) Elements towards the left (cationic-like) of the periodic table uses its elemental name 2) Elements towards the right (anionic-like) of the periodic table substitutes the second half of its elemental name with ide. 3) Use the Greek prefixes to express the number of each element present. (mono, di, tri, tetra, penta, hexa, hepta, octa, etc.) Examples: N2O4 dinitrogen tetraoxide SF6 sulfur hexafluoride PCl3 phosphorus trichloride Note that hydrogen (H2) and oxygen (O2) are also molecules. We do not use prefixes here. 3 Ionic Compounds Ionic Bonding (Ionic Solids): These are formed by a transfer of one or more electrons from one atom to another. Between metal (gives up electrons) and a nonmetal (accepts electrons) 2 Na (s) + Cl2(g) metal nonmetal 2 NaCl (s) (sodium chloride) ionic compound (note: we use (s), (g) and (aq) in chemical equations to denote the physical state of the compound) ionic crystal structure of NaCl Formation of sodium chloride by loss and gaining of electrons 4 Ions in Ionic Compounds Ionic compounds contain charged species called ions. The ions may be charged atoms or molecules but overall the charges cancel to give a neutral compound. For NaCl, Ca(NO3)2 and Al2(SO4)3 the ions are: Cations (+) Anions (-) + Na sodium ion Cl chloride ion Ca2+ calcium ion NO3- nitrate ion Al3+ aluminum ion SO42- sulfate ion Naming Binary Ionic Compounds a) Identify the positive ion and then the negative ion. b) The positive ion uses its elemental name. c) The negative ion substitutes the second half of its elemental name with ide. [note: do not use prefixes] Ions Na+ and Cl combine to form Ca2+ and Br combine to form Al3+ and oxide combine to form Compound NaCl sodium chloride CaBr2 calcium bromide Al203 aluminum oxide Eg. The ion of chlorine is chloride. 5 This document was created with Win2PDF available at http://www.daneprairie.com. The unregistered version of Win2PDF is for evaluation or non-commercial use only.